The present invention relates generally to irrigation control systems, and, more particularly, to systems for the control of relatively large irrigation installations in which there is a master control unit and a number of remotely located control units connected to the master and known as satellite control units. Each satellite unit controls a number of irrigation valves, and, depending on the particular design, some degree of control of the scheduling and timing of the valve operations is retained in the master unit. Master-satellite control systems of this general type have become the accepted technique for the controlled irrigation of relatively large areas, such as golf courses.
Master-satellite systems of the prior art suffer from two principal disadvantages. First, they provide no means for proportionately varying the durations of operation of all of the irrigation valves from the master unit. In general, the duration of irrigation desired at each valve or station will vary according to differing local factors, such as terrain and vegetation. The duration of watering should, however, also be adjusted with changing weather and soil conditions. In irrigation systems of the prior art, an overall change in irrigation duration, e.g., a reduction of 10% at each valve, could only be effected by appropriately adjusting each of a large number of station timers by means of which the durations of irrigation at each station are controlled, or by making an adjustment at each of the satellite units. This is clearly a time-consuming and inefficient method of operation, and is inconsistent with the general philosophy behind a master-satellite system, i.e., to achieve centralized control.
The second disadvantage of master-satellite systems of the prior art relates to the means employed for supplying control signals from the master unit to the satellite units. The stations at each satellite unit are usually activated in a timed sequence. Control signals from the master unit are utilized to start the sequences of operations at the satellites. Once started, each satellite unit is advanced under control of its own program of operations from one station to the next. Control signals from the master unit are also utilized to advance the satellite through all of the stations to a starting or "home" position in the sequence. In some master-satellite systems, each of the satellite units may be designated, by a manual switch on the unit, as an 1 A` or a `B` unit. Two master timing programs are then provided at the master unit, one for the `A` group of satellites and the other for the `B` group, the master timing programs being used to control the generation of the control signals which start the timed sequences of operations at the `A` and `B` groups of satellites. This feature adds flexibility to the system, since it allows for two independent irrigation programs to be in operation, under the control of a single master-satellite system. In this type of system, control signals must be transmitted from the master unit to the satellite units when it is desired to start the satellite units of a particular group, or to reset the units of both groups to the starting position in their programs. Typically, master-satellite systems of the prior art have included separate control lines for the `A` and `B` control signals, and a common return line, thus requiring at least three control lines between the master unit and the satellite units. Since the number and overall length of electrical conductors in irrigation systems is a substantial cost factor, any reduction in the number of control lines represents a significant improvement.
It should be apparent from the foregoing that there is a significant need in the field of irrigation control systems for a master-satellite system which provides a centralized proportional timing control at the master unit, and which reduces the number of control lines or conductors between the master unit and satellite units. The present invention fullfills these needs.